Remove Rails::Engine override to support Rails 4.2

We can just define the initializer inside Propshaft::Railtie.

lib/propshaft/railtie.rb

@@ -2,15 +2,6 @@
 require "rails/railtie"
 require "active_support/ordered_options"
 
-# FIXME: There's gotta be a better way than this hack?
-class Rails::Engine < Rails::Railtie
-  initializer "propshaft.append_assets_path", group: :all do |app|
-    app.config.assets.paths.unshift(*paths["vendor/assets"].existent_directories)
-    app.config.assets.paths.unshift(*paths["lib/assets"].existent_directories)
-    app.config.assets.paths.unshift(*paths["app/assets"].existent_directories)
-  end
-end
-
 module Propshaft
   class Railtie < ::Rails::Railtie
     config.assets = ActiveSupport::OrderedOptions.new
@@ -44,6 +35,12 @@ class Railtie < ::Rails::Railtie
       end
     end
 
+    initializer "propshaft.append_assets_path" do |app|
+      app.config.assets.paths.unshift(*paths["vendor/assets"].existent_directories)
+      app.config.assets.paths.unshift(*paths["lib/assets"].existent_directories)
+      app.config.assets.paths.unshift(*paths["app/assets"].existent_directories)
+    end
+
     initializer "propshaft.logger" do
       Propshaft.logger = config.assets.logger || Rails.logger
     end